How is HP and Torque measured?

[Perry]

When they advertise that our stock motor gets 195hp at 3400rpm and 430ftlbs ot torque at 1800rpm, how is that determined?

When i'm cruising on a level highway with the engine turning 1800rpm, is it really pushing out 430ftlbs of torque, even when there is barely any boost? Or is that figure only when on the throttle at max boost when at 1800rpm?

The reason I ask is that published on this dieselpage website in the section titled Diesel Economic and Fuel Mileage, it states that "A typical 6.5TD with 3.42 gearing will produce in excess of 20 mpg at 65 mph because the engine rpm at that speed is right at the engine's torque peak of about 1800 rpm"

But if you are just cruising at 65mph, are you really at the engine's torque peak even at light throttle? And why is it at the torque peak that the motor is most efficient? So with that statement published on this website, it could also mean that if you cruise at less than 1800rpm, your mileage will be worse which i don't think is the case.

Sorry for bringing this up...with fuel prices at $3.00 fuel efficiency is always on my mind, and I'm starting to find myself going the speed limit in the right lane.

[ronniejoe]

The advertised numbers are for maximum effort conditions. When you are cruising on the highway, you are using much less than maximum effort from the engine to overcome wind drag, rolling resistance and other losses. The torque peak rpm is usually near where the engine is most efficient, at any power level.

[6.6/6.5]

Perry, Ron's response is correct --- peak torque is the maximum output of the engine. Basically, engine torque is measured by a torsional load cell (technically a calibrated Whetstone electronic bridge) on the input shaft of the dynamometer being used to measure the engine's output. Engine speed (RPM) is also measured. Maximum torque (measured in lb.-ft. for this example) is recorded for the full rpm range from slightly above low idle (1000 RPM is a good starting point) through the maximum RPM (either a pre-established "red line" engine speed or the maximum governed engine speed). From the resulting curve, peak torque at the corresponding RPM will be readily apparent --- the highest torque level recorded --- and the corresponding RPM. Horsepower isn't measured directly, it is calculated from each torque/RPM data point: HP = Torque (measured in lb.-ft.) X RPM (revolutions per minute) / 5252. Another way of representing this relationship is HP = T x N/5252 where T is torque and N is RPM. (The constant 5252 corrects for the units of measurement, thus it only works for Horsepower, Torque in lb. ft. and revolutions per minute.) With the power curve being based on each data point from the torque and RPM as measured on the dyno, maximum or peak power is simply taken from the resulting power curve. I hope it is obvious that the power curve can be calculated for any engine for which the torque/RPM dynamometer data exist. I hope that Ron's reponse and my explanation of how engine horsepower is determined answer your question.